1. Field of the Invention
The present invention relates to an ignition system for an internal combustion engine, more particularly, to an ignition system for an internal combustion engine of so-called two cylinder simultaneous ignition type, in which a plurality of ignition coils are connected to respective spark plugs in respective engine cylinders and, among the ignition coils, primary windings of two ignition coils connected to the spark plugs of the cylinders working in pairs in compression stroke and exhaust stroke, are connected in series for simultaneous exciting the primary windings.
2. Description of the Related Art
Conventionally, there have been known ignition systems, in which high tension cords are eliminated by connecting each spark plug in the internal combustion engine to the ignition coil directly, and number of switching elements or power transistors for excitation control is reduced to be a half of number of cylinders of the internal combustion engine by connecting in series the primary windings connected to the spark plugs of two cylinders which are correlated in timing so that one is in the compression stroke while the other is in the exhaust stroke, for simultaneous excitation. (See Journal of Nippondenso Technical Disclosure 34-61, 82-103).
The foregoing ignition system is a so-called inductive discharge type ignition system which flows current through the primary windings of two ignition coils by directly applying power source voltage or battery voltage and produces sparks through the spark plugs by a high voltage generated on secondary windings upon shutting off of the current. Because two primary windings are connected in series, the excitation period for the primary windings to obtain a given excitation current for generating the high voltage at the secondary winding sufficient for ignition, becomes substantially double of the case where the primary winding for a single ignition coil is to be excited. Therefore, in the above-mentioned ignition system, a problem is encountered in that the voltage to be generated in the secondary winding is significantly lowered in a high speed range of the internal combustion engine where the available period gets short for excitation of the primary winding.
Also, when the primary windings of two spark plugs are excited simultaneously, a problem may be encountered in that since an ignition energy is accumulated in the ignition coil for discharging, the necessary energy for exciting two primary windings becomes double of the case where a single winding of the ignition coil is to be excited.
On the other hand, a system for simultaneously exciting primary windings of two ignition coils which are connected in series for simultaneously sparking two spark plugs, is disclosed in Japanese Examined Utility Model Publication JP-Y-50-28281, for example. Therefore, an ignition system of a so-called capacitive discharge type is also known, in which a high voltage is preliminarily charged in a capacitor before exciting the ignition coils and the primary winding is excited with the high voltage charged in the capacitor at a given ignition timing.
This ignition system is a system for simultaneously charging an igniting high voltage for two spark plugs in a single cylinder. Such capacitive discharge type system instantly generates the high voltage on the secondary windings of the ignition coil by exciting the primary windings of the ignition coil with the high voltage charged in the capacitor. As set forth above, applying this system to the two cylinder simultaneous ignition type ignition system, in which two ignition coils connected to the spark plugs of two cylinders forming a pair of the compression stroke and exhaust stroke, the above problem in that the voltage generated in the secondary winding may be lowered at the high speed range of the internal combustion engine for prolongation of the excitation period for the primary windings, can be avoided. Therefore, the high voltage can be quickly generated on the secondary winding.
However, even in such capacitive discharge type ignition system, it is required substantially higher energy for simultaneously exciting the primary windings of two ignition plugs connected in series, than that for exciting the primary winding of a single spark plug as disclosed by Morino et al in U.S. Pat. No. 5,056,496. Occasionally, it may be required double of energy to that in the case where the primary winding of the single spark plug, as in the induction discharge type.